1. After overnight deprivation of water both the cellular and extracellular fluid volumes are significantly reduced in the rat. 2. In the rat with functional kidneys oral, intragastric or intravenous preloads of 10 ml. water reduce the total water intake after 1 hr by 64-69%. These preloads restore plasma osmolality to pre-deprivation levels but have little effect on plasma volume. 3. In the same rats if the plasma volume is restored with an oral, intragastric or intravenous preload of 10 ml. of an isotonic balanced salt solution which has little effect on osmolality, drinking is significantly reduced by 20-26%. The reduction of drinking correlated with the volume of the preload of balanced salt. 4. Plasma analysis shown that 1 hr after an oral preload of 10 ml. isotonic balanced salt solution, the extracellular fluid volume of the deprived rats is restored to pre-deprivation levels but osmolality is unchanged. Three hr after the balanced salt preload, extracellular fluid volume is still at pre-deprivation levels and there has been a slight decrease in osmolality due to excretion of salt. 5. In rats which had been nephrectomized or had the ureters ligated so there could be no renal modification of the preloads, the effects of the preloads of water and balanced salt are the same as in rats with intact kidneys. 6. The results indicate that after water deprivation in the rat, changes in both the cellular and extracellular fluid compartment are stimuli to drinking.
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